1. Field of the Invention
The present invention relates generally to prosthetic devices for replacing injured or diseased natural members and, more particularly, to such devices provided with porous surfaces for promoting bone ingrowth fixation.
Although the invention need not be so limited, it is disclosed in relation to a hip prosthesis adapted for insertion into the upper medullary canal in the femur of a patient. The hip prosthesis of this invention is of the type generally characterized as including a head or ball member, a shaft or stem member for insertion into the intramedullary canal, and a neck member connecting the head and stem. The prosthesis also includes at least one porous metal surface portion which provides for stabilization by bone ingrowth fixation without requiring any cement.
2. Description of the Prior Art
Hip prostheses are known in the art and these have included various design configurations of the various components, including the head member, neck, collar and stem. In recent years, prosthetic devices have been provided with porous surfaces for bone ingrowth fixation. An excellent example of such a surface results from application of the proprietary porous metal coating of DePuy Division of Boehringer Mannheim Corporation provided under the trademark "POROCOAT". Representative of patents in this field include U.S. Pat. Nos. 3,605,123 to Hahn, No. 3,855,638 to Pilliar, No. 4,536,894 to Galante et al, and No. 4,854,496 to Bugle.
By providing a bone ingrowth surface on the prosthetic device, a stable fixation can be achieved without the use of bone cement. However, even with prosthetic devices provided with a bone ingrowth fixation surface or surfaces, sufficient bone ingrowth fixation to provide long term stabilization requires that the prosthesis be stably fixed without movement for a period of time post implantation, and any excessive relative motion between the bone and the prosthesis during that period can prevent or compromise such fixation. This is a particularly significant problem in view of the difficulty of fitting the prosthesis with sufficiently close tolerances to provide large contact areas between the porous material and the bone, even where the entire outer surface of the prosthesis is fabricated from porous material.
It is generally understood that the success of a cementless joint implant arthroplasty depends upon the initial stability of the implant following its insertion into the skeletal member. The stability of the implant is, in turn, determined essentially by two factors: (1) the geometry or shape of the implant and (2) the surface roughness of the implant. Regarding the latter, it is desirable to porous coat all of the implant surface which is in apposition with the bone. Essentially, the increased surface area provided by the porous coating results in an exceedingly strong friction fit between stem and bone which promotes implant stability improving the likelihood of bone ingrowth fixation for long term fixation. The stability of the implant may be even further enhanced by providing an interference fit between the outer surface of the porous coating and the bone by allowing the porous coated region to protrude beyond the boundary formed by the smooth outer surface of the implant.
It is a known phenomenon, though, that the porous coating can act as a notch, that is, a surface defect on the surface of the implant. Certain preferred materials for prostheses such as the titanium alloy Ti-6Al-4V are "notch sensitive" and can experience a drastic reduction in fatigue strength when the porous coating is located in high stress regions of the implant. Typically, the fatigue strength of titanium alloys can be reduced by approximately 70% when porous coated.
Also, it has been postulated with some degree of clinical support that stemmed implants with very thick sections and a resultant high bending stiffness may have a deleterious effect on the viability of the bone into which it is placed. The stiff stem, in effect, stress protects the surrounding bone. If bone is not regularly stressed to within certain limits the bone will atrophy until the stress level rises to a value within the desired range. In the case of the thickest and stiffest stems the amount of atrophy can be severe. This phenomenon and other approaches for addressing the problem have been disclosed in U.S. Pat. No. 4,808,186 and in U.S. application Ser. No. 391,660 filed Aug. 8, 1989.
The purposes of the present invention are three-fold. A first purpose is to reduce the section modulus and resulting bending stiffness of thick stems to reduce the amount of bone stress protection. A second purpose is to porous coat significant surface areas of the stem in such as way as to avoid notching the high stress regions yet still provide for a tight friction, even interference, fit of the stem within the intramedullary canal. A third purpose is to obtain a maximum surface area for the porous coating to thereby obtain the strongest possible bond between the bone and the prosthesis.
When a femoral hip implant is loaded, the stem is typically placed in a bending mode of stress. In the bending mode of loading the highest stresses exist on the most lateral outer fibers of the stem. The stresses decrease to zero in a linear fashion toward the center or longitudinal axis of the stem which is therefore referred to as the neutral axis. A porous coating located at the outer fiber high stress region would unfortunately operate to notch the implant substrate and can likely lead to premature failure. According to the invention, however, channels are cut into the stem in a longitudinal direction. The porous coating is attached to the bottom of the channels only. In this manner, the fiber stresses are lower to an extent primarily dependent upon the depth of the channel. Notching the high stress region is thereby avoided. The porous coating can be built up to a thickness to be generally level with the outer surface of the implant. Therefore, a portion of the desired friction fit in the stem region of an implant can be achieved.
The longitudinal channels also serve to reduce the section modulus of the stem yielding it more flexible. This maintains the viability of the bone in the stem region by reducing the amount of stress protection usually caused by large solid stems.
Still another benefit of the invention resides in the significantly increased surface area thereby provided by the porous coating for enhanced bone ingrowth fixation. That is, the porous coating applied in the manner of the invention does not only present its usual outer surface lying generally within or parallel to an outer surface of the stem. It also presents two additional surfaces which lie in radial planes generally parallel to but spaced from the sidewalls of the channel.
Other and further features, advantages, and benefits of the invention will become apparent in the following description taken in conjunction with the following drawings. It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory but are not to be restrictive of the invention. The accompanying drawings which are incorporated in and constitute a part of this invention, illustrate one of the embodiments of the invention, and, together with the description, serve to explain the principles of the invention in general terms. Like numerals refer to like parts throughout the disclosure.